1. Field of the Invention
The present invention relates to a method of reducing the trap density of an oxide film formed in a semiconductor device and, more particularly, to a method of reducing the trap density of an oxide film formed in a semiconductor device suitable for application to fabricating a nonvolatile memory cell.
2. Description of the Related Art
An EEPROM of a floating gate construction having a floating gate electrode formed under an insulating layer (oxide film) underlying a control gate electrode is a known nonvolatile memory cell.
This EEPROM has a thin oxide film (tunnel region) between a drain and a floating gate electrode. Electrons are injected through the thin oxide film (tunnel oxide film) into the floating gate or electrons are removed from the floating gate through the thin oxide film to erase or write information.
In writing or erasing information, an electric field of a high intensity is applied to the thin tunnel oxide film to allow electron to tunnel through the thin tunnel oxide film. Therefore, it is essential that the thin tunnel oxide film is capable of withstanding repeated application of a high-intensity electric field thereto to enhance the endurance characteristics (which are represented by the number of possible data rewriting cycles) of the EEPROM.
The endurance characteristics may be improved if the breakdown of the tunnel oxide film attributable to the local concentration of the high-intensity electric field applied to the EEPROM in rewriting data can be prevented and it is considered that the endurance characteristics can be effectively improved by forming the tunnel oxide film in a uniform film quality.
A high-temperature annealing in an N.sub.2 atmosphere at a high temperature on the order of 1000.degree. C. is a means for improving the film quality of an oxide film.
However, it was found through experiments conducted by the inventors of the present invention that the tunnel oxide film is damaged and trap sites are formed by a plasma CVD process for forming a silicon nitride (SiN) film having an excellent hygroscopic property as a passivation film.
Thus, if the tunnel oxide film has trap sites, electrons are liable to be trapped when tunnel current flows through the tunnel oxide film in rewriting data in the EEPROM, the intensity of the internal electric field of the tunnel oxide film increases with the increase of the trapped electrons and, finally, the breakdown of the tunnel oxide film occurs.
Since the tunnel oxide film damaged in forming the passivation film is repaired after an aluminum electrode has been formed, it is impossible to improve the quality of the tunnel oxide film by high-temperature annealing.